Thermal interface material and method for making the same

ABSTRACT

A thermal interface material includes a matrix and a plurality of carbon nanotubes. The matrix includes a first surface and an opposite second surface. The carbon nanotubes are embedded in the matrix uniformly. The carbon nanotubes extend from the first surface to the second surface and each have two opposite ends. At least one of the two opposite ends of the carbon nanotubes is exposed out of one of the first and second surfaces of the matrix. The exposed ends of the carbon nanotubes being elastically bent/embedded in a phase change layer formed thereon.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to three corresponding U.S. patent applications as follows: application Ser. No. 11/024513, filed on Dec. 28, 2004, entitled “THERMAL INTERFACE MATERIAL AND METHOD FOR MAKING SAME”, application Ser. No. 11/025160, filed on Dec. 28, 2004, entitled “METHOD FOR MAKING CARBON NANOTUBES WITH UNIFORM LENGTH”, application Ser. No. 11/089864, filed on Mar. 25, 2005, entitled “THERMAL INTERFACE MATERIAL AND METHOD FOR MAKING SAME”, a recent application entitled “THERMAL INTERFACE MATERIAL AND METHOD FOR MAKING THE SAME”, and a recent application entitled “THERMAL INTERFACE MATERIAL AND METHOD FOR MANUFACTURING SAME”, each having the same assignee as the instant application. The disclosures of the above-identified applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The invention relates generally to thermal interface materials and making methods thereof; and more particularly to a thermal interface material which conducts heat by using carbon nanotubes, and a making method thereof.

2. Discussion of the Related Art

Electronic components such as semiconductor chips are becoming progressively smaller, while at the same time heat dissipation requirements thereof are increasing. Commonly, a thermal interface material is utilized between the electronic component and a heat sink in order to efficiently dissipate heat generated by the electronic component.

A conventional thermal interface material is made by diffusing particles with a high heat conduction coefficient in a base material. The particles can be made of graphite, boron nitride, silicon oxide, alumina, silver, or other metals. However, a heat conduction coefficient of the thermal interface material is now considered to be too low for many contemporary applications, because it cannot adequately meet the heat dissipation requirements of modem electronic components.

A new kind of thermal interface material has recently been developed. The thermal interface material is obtained by fixing carbon fibers with a polymer. The carbon fibers are distributed directionally, and each carbon fiber can provide a heat conduction path. A heat conduction coefficient of this kind of thermal interface material is relatively high. However, the thickness of this kind thermal interface material is limited to be greater than 40 micrometers, and the heat conduction coefficient of the thermal interface material is inversely proportional to a thickness thereof. In other words, the heat conduction coefficient is limited to a certain value corresponding to a thickness of 40 micrometers. The value of the heat conduction coefficient cannot be increased, because the thickness cannot be reduced.

An article entitled “Unusually High Thermal Conductivity of Carbon Nanotubes” and authored by Savas Berber (page 4613, Vol. 84, Physical Review Letters 2000) discloses that a heat conduction coefficient of a carbon nanotube can be 6600 W/mK (watts/milliKelvin) at room temperature.

U.S. Pat. No. 6,407,922 discloses another kind of thermal interface material. The thermal interface material is formed by injection molding, and has a plurality of carbon nanotubes incorporated in a matrix material. A first surface of the thermal interface material engages with an electronic element, and a second surface of the thermal interface material engages with a heat sink. The second surface has a larger area than the first surface, so that heat can be uniformly spread over the larger second surface.

However, the thermal interface material formed by injection molding is relatively thick. This increases a bulk of the thermal interface material and reduces its flexibility. Furthermore, the carbon nanotubes are disposed in the matrix material randomly and multidirectionally. This means that heat does not necessarily spread uniformly through the thermal interface material. In addition, the heat does not necessarily spread directly from the first surface engaged with the electronic element to the second surface engaged with the heat sink.

What is needed, therefore, is a thermal interface material that has a low thermal interface resistance and a high heat conducting efficiency.

What is also needed, therefore, is a method for making the above-described thermal interface material.

SUMMARY

In accordance with an embodiment, a thermal interface material includes a matrix and a plurality of carbon nanotubes embedded in the matrix. The matrix has a first surface and an opposite second surface. The carbon nanotubes extend from the first surface to the second surface and each having two opposite ends. The at least one of the two ends of each carbon nanotube is exposed out of one of the first and second surfaces of the matrix. The exposed end of each carbon nanotube is elastically bent/embedded in a phase change layer formed thereon.

In accordance with another embodiment, a method for making the thermal interface material includes the steps of:

-   (a) providing a plurality of carbon nanotubes having two opposite     ends; -   (b) forming a protective layer on at least one of the two ends of     the carbon nanotubes; -   (c) injecting curable liquid matrix into clearances among the carbon     nanotubes and curing the liquid matrix; -   (d) removing the protective layer to expose at least one of the two     ends of the carbon nanotubes; and -   (e) elastically bending/embedding the exposed end of each carbon     nanotube in a phase change layer formed thereon.

Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the thermal interface material can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present thermal interface material. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a perspective view of a thermal interface material according to a preferred embodiment;

FIG. 2 is a schematic side view of the thermal interface material of FIG. 1;

FIG. 3 illustrates a method for making the thermal interface material of FIG. 2;

FIG. 4 is a schematic, cross-section view of the thermal interface material of FIG. 2 applied between an electronic element and a heat sink.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present thermal interface material will now be described in detail below and with reference to the drawings.

Referring to FIGS. 1 and 2, a thermal interface material (TIM) 10 includes a matrix 2, a plurality of carbon nanotubes (CNTs) 1, and dual phase change layers 3. The matrix 2 has a first surface 21 and an opposite second surface 22. The CNTs 1 are embedded in the matrix 2 uniformly. The CNTs 1 each include a carbon nanotube body submerged into the matrix 2 and two opposite ends, i.e., a first end 11 and an opposite second end 12. The two opposite ends 11 and 12 are exposed out of the first and second surfaces 21, 22 of the matrix 2, respectively. The dual phase change layers 3 are formed on the two ends 11 and 12, respectively.

The first and second surfaces 21, 22 of the matrix 2 are substantially parallel to one another. The matrix 2 preferably has a thickness in the range from 1 to 1000 micrometers. In the preferred embodiment, the thickness of the matrix 2 is 20 micrometers. The matrix 2 is comprised of, for example, a macromolecular material selected from the group consisting of silicone elastomer, polyester, polyvinyl chloride, polyvinyl alcohol, polyethylene, polypropylene, epoxy resin, polycarbonate resin, polyoxymethylene, and polyacetal.

The CNTs 1 are beneficially in a form of an aligned CNTs array. The carbon nanotube bodies are substantially parallel to each other. Further, the carbon nanotube bodies are substantially perpendicular to the first surface 21 and the second surface 22. Thus, each carbon nanotube 1 can provide a heat conduction path in a direction perpendicular to a selected main heat-absorbing surface of the TIM 10.

In the illustrated embodiment (e.g., see FIG. 2), each carbon nanotube 12 extends out of the first and second surfaces 21, 22 of the matrix 2, thereby exposing the first and second ends 11, 12. Alternatively, one of the first and second ends 11, 12 of the CNTs 1 extends out of the first and second surfaces 21, 22 of the matrix 2, thereby exposing one of the first and second ends 11, 12. In that case, only a phase change layer 3 is formed on the exposed corresponding end, i.e., the first or second ends 11, 12.

The dual phase change layers 3 are formed on the first and second surfaces 21, 22, respectively. The dual phase change layers 3 are configured, for example, for filling the clearances 112, 122 between the exposed first and second ends 11, 12 of adjacent CNTs 1. The first and second ends 11, 12 are elastically bent/embedded in a respective phase change layer 3. The bending direction of the ends of the CNTs could be in a consistent or random direction. The two ends 11, 12 of a single carbon nanotube 1 could be elastically bent/embedded in the dual phase change layer 3 in a different direction. Further, some ends of the CNTs 1 could be partially bent/embedded in the phase change layers 3.

Each phase change layer 3 preferably has a thickness in the range from 1 to 100 micrometers. In the preferred embodiment, the thickness of each phase change layer 3 is 10 micrometers. Each phase change layer 3 is comprised of, e.g., a phase change material selected from the group consisting of paraffin, polyolefin, low molecular polyester, low molecular epoxy resin, and low molecular polyacrylate. Each phase change layer 3 preferably has a desirable phase change temperature, which is corresponding to an operating temperature of a heat-generating element (e.g., an electronic element), for example, in the range from about 20° C. to about 90° C.

FIG. 3 illustrates a method for making the above-described TIM 10. In the illustrated embodiment, the method includes the following steps:

-   (a) providing a plurality of CNTs 1 having two opposite ends 11, 12; -   (b) forming two protective layers 7 on the two ends 11, 12 of the     CNTs 1; -   (c) injecting curable liquid matrix 2 into clearances among the CNTs     1 and curing the liquid matrix 2; -   (d) removing the protective layer 7 to expose the two ends 11, 12 of     the CNTs 1; -   (e) elastically bending/embedding the exposed ends 11, 12 of each     carbon nanotube 1 in a corresponding phase change layer 3 formed     thereon.

In step (a), the CNTs 1 is formed by a method selected from the group consisting of a chemical vapor deposition (CVD) method, a plasma-enhanced chemical vapor deposition (PECVD) method, and a hot-filament chemical vapor deposition (HFCVD) method. In the preferred embodiment, the CNTs 1 is formed by a CVD method, referring to an article entitled “Self-Oriented Regular Arrays of CNTs and Their Field Emission Properties”, science, 1999, Vol. 283, 512-514.

In brief, a substrate 4 (e.g., a silicon substrate) is coated with Fe film of 5 nm thick and then annealed in air at 300° C. The growth of CNTs 1 is then performed in a CVD chamber and using ethylene as feed gas at 700° C. As shown in FIG. 3 a, the CNTs 1 are formed on the substrate 4. The CNTs 1 have a first end 11 and an opposite second end 12. The CNTs 1 are substantially parallel to one another. The CNTs 1 could be controlled in a predetermined highness in accordance with the thickness of the matrix 2 by adjusting reaction time of the feed gas. The CNTs advantageously have an outer diameter of about 12 nm. The substrate 4 could be gently stripped away from the CNTs 1 thereby obtaining the isolating CNTs 1.

In step (b), briefly, a pressure sensitive adhesive layer 6 is coated on a supporting film 5 comprised of a polymer (e.g., polyester) thereby forming a protective layer 7. Then the protective layer 7 is formed on the first end 11 (see FIG. 3 a) of as-prepared CNTs 1 by gently pressing the pressure sensitive adhesive layer 6 adjacent the matrix 2, thereby securing the first end 11 of the CNTs 1 in the protective layer 7. Further, another protective layer 7 could be formed on the second end 12 (see FIG. 3 a) of the CNTs 1 by a similar step to the step (b). As such, the two ends 11, 12 of the CNTs 1 are secured in a corresponding protective layer 7, as shown in FIG. 3 b. The protective layer 7 advantageously has a thickness of about 0.05 mm.

Alternatively, after the formation of the protective layer 7 on the first end of the CNTs 1, the CNTs 1 is directly conducted to the next step. In that condition, only the first end 11 of the CNTs 1 is covered by the protective layer 7 and thereby extending out of a corresponding surface of the matrix 2 according to the following steps, instead.

In step (c), the injection is performed by the step, for example, of submerging the CNTs 1 into the liquid matrix 2, and then curing the liquid matrix 2 in a vacuum chamber at room temperature for 25 hours. As shown in FIG. 3 c, the cured matrix 2 thereby is inserted into clearances among the adjacent CNTs 1. The liquid matrix 2 is advantageously a polymer solution comprised of the above-mentioned matrix material in the TIM 10. Taking a silicone elastomer for an example, the liquid matrix 2 includes a S160 solution (i.e., Sylgard 160 produced by Dow Corning) and ethyl acetate at a volume ratio of 1:1.

In step (d), the protective layer 7, i.e., the supporting film 5 and pressure sensitive adhesive layer 6, is removed thereby exposing the first and second ends 11, 12 of the CNTs 1 out of the first and second surfaces 21, 22 of the matrix 2. The protective layer 7 may be removed, for example, by directly stripping off the supporting film 5 and sequentially dissolving away the remaining pressure sensitive adhesive layer 6 in a xylene solution. Thus, a composite film of the CNTs 1 embedded in the S160 matrix 2 can be obtained, as shown in FIG. 3 d. After step (d), the outline of the CNTs of the composite film is essentially the same with the original CNTs. That is, the CNTs 1 are still aligned in the S160 matrix.

Further treatments on the first and/or second surfaces 21, 22 of the matrix 2 could be optionally conducted by reactive ion etching (RIE) to ensure all CNT ends revealed. The RIE processes could be carried out, for example, by using oxygen plasma at a pressure of 6 Pa and with a power of 150 W for 15 minutes at each surface of the matrix 2.

In step (e), the exposed ends of each carbon nanotube could be elastically bent/embedded in a respective phase change layer, e.g., by a method selected form the group consisting of a pressing method, a polishing method, a rubbing/scraping method, and a cutting method.

The pressing method beneficially includes the steps of: forming a phase change layer 3 on two surfaces of two panels respectively; sandwiching the CNTs 1 embedded into the matrix 2 between the two surfaces of the two panels; pressing and heating the phase change layer so as to bend/embed the exposed ends of each carbon nanotube thereinto; and curing the phase change layers and removing the two panels thereby forming the thermal interface material.

In pressing and heating step, the heating temperature is at/above a phase change temperature of the phase change layer 3 thereby melting the phase change layer 3. Thus, the exposed end of each carbon nanotube is readily elastically bent/embedded into the phase change layer 3 due to a pressing action. In this case, the exposed ends of the carbon nanotubes are elastically bent/embedded into the phase change layer 3 in a random diretion. Because the CNTs have springback effect, the pressed end of each carbon nanotube would spring back and beneficially extend out of the phase change layer 3 once the phase change layer 3 is melted.

The polishing method could be performed, for example, in a chemical mechanical polishing machine or a grinding and polishing machine. In brief, the polishing method includes the steps of: admixing a phase change material into an abrasive; and polishing the exposed ends of the carbon nanotubes by using the abrasive with the phase change material. Alternatively, the polishing method may include the steps of: forming a phase change layer on the exposed ends of the carbon nanotubes; polishing the exposed ends of the carbon nanotubes so as to bend/embed the exposed ends of the carbon nanotubes thereinto. During the two polishing step, the phase change layer/material is heated to/above the phase change temperature, and then is cured so as to elastically bend/embed the exposed ends of the carbon nanotubes thereinto.

The rubbing/scraping method includes the steps of: forming a phase change layer 3 on the exposed ends of the carbon nanotubes 1 and allowing the exposed ends of the carbon nanotubes 1 to extend out of the phase change layer 3; and rubbing/scraping the extending portions of the exposed ends of the carbon nanotubes with a rough substrate one another to bend/embed the exposed ends of the carbon nanotubes into the phase change layer. The formation of each phase change layers 3 on the exposed ends of the carbon nanotubes 1 is performed, for example, by directly adhering a phase change material sheet on the corresponding surface of the matrix 2 below the phase temperature. Alternatively, the formation of each phase change layers 3 is performed, e.g., by the following steps of: submerging the corresponding surface of the matrix 2 into a liquid phase change material solution, and then adsorbing and thus removing the undesirable liquid phase change material by using a filter paper.

After the formation of the phase change layer 3 on the exposed ends of the carbon nanotubes 1, the exposed ends are further rubbed/scraped to bend/embed the exposed ends into the phase change layer. In brief, the rubbing/scraping step could be performed, for example, by moving the exposed ends of the carbon nanotubes 1 against the still rough substrate, or by moving the rough substrate against the exposed ends of the carbon nanotubes 1. The bending direction of the exposed ends depends on the rubbing/scraping direction of the rough substrate or the exposed ends.

The cutting method includes the steps of: submerging the carbon nanotubes embedded into the matrix into a liquid phase change material; curing the liquid phase change material; and cutting the cured phase change material formed on the exposed ends of the carbon nanotubes by using a cutter in a direction perpendicular to long axes of the carbon nanotubes. The cut end of each carbon nanotube could be open and is elastically bent/embedded into the cured phase change layer. The bending direction of the ends depends on the cutting direction of the cutter.

In addition, by decreasing the thickness of each phase change layers 3 formed on the ends of the carbon nanotubes 1, the ends 11 and/or 12 of the CNTs could spring back and further extend out of the surface of the corresponding phase change layer 3 once the phase change layer 3 is at/above the phase change temperature.

While only one of the two ends 11, 12 of the CNTs 1 is covered by the protective layer 7, the end 11/12 of the CNTs 1 is correspondingly exposed out of the corresponding surface 21/22 of the matrix 2 after the above-described step (c) and (d). Accordingly, the other end 12/11 of the CNTs 1 is submerged into the cured matrix 2. Thus, in step (e), only one phase change layer 3 is required to bend/embed the exposed end 11/12 of the CNTs 1 thereinto.

FIG. 4 illustrates an application of the TIM 10. The TIM 10 is sandwiched between a heat sink 8 and a heat-generating element 7, for providing good heat contact between the heat sink 8 and the heat-generating element 7. In operation, the heat-generating element 7 generates heat, which is immediately conducted to the phase change layer 3. Then, the phase change layer 3 is melted thereby allowing the bent end (i.e., the second end 12) to extend out of the phase change layer 3 by the springback effect of the carbon nanotubes. The springback end 12 could form a partial pressure on a topside 71 of heat-generating element 7 due to an inner stress of the oppressed carbon nanotube 1. Next, heat is conducted to another phase change layer 3 formed on the first end 11. Similarly to the second end 12, the first end would extend out of the respective phase change layer 3 and then forms a partial pressure on an underside 81 of heat sink 7.

The partial pressures of the ends 11, 12 of the carbon nanotubes 1 can ensure that the carbon nanotubes 1 have excellent heat contact with the heat-generating element 7 and the heat sink 8, thereby decreasing heat resistance between heat conducting interfaces. Furthermore, the pressures of the two ends 11, 12 are generally above 3 MPa. Thus, a fastening force, which is required for packing the TIM and is generally about 0.5 MPa, would be decreased or even needless by directly utilizing the pressures of the two ends 11, 12.

The phase change layers 3 are capable of filling the clearances 112, 122 between the exposed first and/or second ends 11, 12 of adjacent CNTs 1. Thus, the thermal interface resistances induced by the clearances 112, 122 are decreased. As a result, the heat conducting efficiency of the TIM 10 is further enhanced.

It is understood that the above-described embodiments and methods are intended to illustrate rather than limit the invention. Variations may be made to the embodiments and methods without departing from the spirit of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. 

1. A thermal interface material comprising: a matrix having a first surface and an opposite second surface; and a plurality of carbon nanotubes embedded in the matrix, the carbon nanotubes extending from the first surface to the second surface and each having two opposite ends, at least one of the two ends of the carbon nanotubes being exposed out of one of the first and second surfaces of the matrix, the exposed ends of the carbon nanotubes being elastically bent/embedded in a phase change layer formed thereon.
 2. The thermal interface material as claimed in claim 1, wherein one of partial and whole exposed ends of the carbon nanotubes are elastically bent/embedded in a phase change layer formed thereon.
 3. The thermal interface material as claimed in claim 1, wherein the exposed ends of the carbon nanotubes are configured to extend out of the phase change layer at/above a phase change temperature thereof.
 4. The thermal interface material as claimed in claim 1, wherein the phase change layer has a thickness in the range from 1 to 100 micrometers.
 5. The thermal interface material as claimed in claim 1, wherein the phase change layer is comprised of a phase change material selected from the group consisting of paraffin, polyolefin, low molecular polyester, low molecular epoxy resin, and low molecular poly acrylate.
 6. The thermal interface material as claimed in claim 5, wherein the phase change material has a phase change temperature in the range from about 20° C. to about 90° C.
 7. The thermal interface material as claimed in claim 1, wherein the carbon nanotubes have carbon nanotube bodies wholly submerged in the matrix, the carbon nanotube bodies are substantially parallel to each other.
 8. The thermal interface material as claimed in claim 1, wherein the first and second surfaces of the matrix are substantially parallel to each other.
 9. The thermal interface material as claimed in claim 8, wherein the carbon nanotube bodies are substantially perpendicular to the first and second surfaces of the matrix.
 10. The thermal interface material as claimed in claim 1, wherein the thermal interface material has a thickness in the range from 1 to 1000 micrometers.
 11. The thermal interface material as claimed in claim 1, wherein the matrix is comprised of a material selected from the group consisting of silicone elastomer, polyester, polyvinyl chloride, polyvinyl alcohol, polyethylene, polypropylene, epoxy resin, polycarbonate resin, polyoxymethylene, and polyacetal.
 12. A method for making a thermal interface material, the method comprising the steps of: providing a plurality of carbon nanotubes having two opposite ends; forming a protective layer on at least one of the two ends of the carbon nanotubes; injecting curable liquid matrix into clearances among the carbon nanotubes and curing the liquid matrix; removing the protective layer to expose at least one of the two ends of the carbon nanotubes; and elastically bending/embedding the exposed ends of the carbon nanotubes in a phase change layer formed thereon.
 13. The method according to claim 12, wherein the exposed ends of the carbon nanotubes are elastically bent/embedded in the phase change layer by a method selected form the group consisting of a pressing method, a polishing method, a rubbing/scraping method, and a cutting method.
 14. The method according to claim 13, wherein the pressing method comprises the steps of: forming a phase change layer on two surfaces of two panels respectively; sandwiching the carbon nanotubes embedded into the matrix between the two surfaces of the two panels; pressing and heating the phase change layer so as to bend/embed the exposed end of each carbon nanotube thereinto; and curing the phase change layer and removing the two panels thereby forming the thermal interface material.
 15. The method according to claim 13, wherein the polishing method comprises the steps of: admixing a phase change material into an abrasive; and polishing the exposed ends of the carbon nanotubes by using the abrasive with the phase change material.
 16. The method according to claim 13, wherein the polishing method comprises the steps of: forming a phase change layer on the exposed ends of the carbon nanotubes; polishing the exposed ends of the carbon nanotubes so as to bend/embed the exposed ends of the carbon nanotubes thereinto.
 17. The method according to claim 13, wherein the rubbing/scraping method comprises the steps of: forming a phase change layer on the exposed ends of the carbon nanotubes and allowing the exposed ends of the carbon nanotubes to extend out of the phase change layer; and rubbing/scraping the extending portions of the exposed ends of the carbon nanotubes with a rough substrate one another to bend/embed the exposed ends of the carbon nanotubes into the phase change layer.
 18. The method according to claim 13, wherein the cutting method comprises the steps of: submerging the carbon nanotubes embedded into the matrix into a liquid phase change material; curing the liquid phase change material; and cutting the cured phase change material formed on the exposed ends of the carbon nanotubes by using a cutter in a direction perpendicular to long axes of the carbon nanotubes.
 19. A thermal management system comprising: a heat source; a heat sink; and a thermal interface material sandwiched between the heat source and the heat sink, the thermal interface material comprising: a matrix having a first surface and an opposite second surface; and a plurality of carbon nanotubes embedded in the matrix, the carbon nanotubes extending from the first surface to the second surface and having two opposite ends, at least one of the two ends of the carbon nanotubes being exposed out of one of the first and second surfaces of the matrix, the exposed ends of the carbon nanotubes being elastically bent/embedded in a phase change layer formed thereon. 